1. Field
This invention relates to X-ray machines, specifically the tube head assemblies of such machines. It is specifically directed to a resilient container for the cooling oil and high-voltage subassembly of a lightweight tube head assembly.
2. State of the Art
The tube head assemblies of X-ray machines used within the dental profession typically include an X-ray tube, means to generate the voltages required to power the X-ray tube, lead shielding to prevent radiation from leaking from the assembly, and a housing which serves both as a rigid mounting structure for the other components and as a container for cooling medium, typically transformer cooling oil A transformer is used to step up and step down, as required, a source voltage to those required by the X-ray tube. In dental X-ray machines, a voltage of approximately 65,000 volts may be applied across the tube electrodes while a voltage on the order of 3 volts may be applied to the tube filament. It has become conventional practice to contain the cooling medium within an enclosure having pressure responsive capability. The capability is provided by a bellows, sleeve, or diaphragm of elastomeric resilient material associated with the enclosure. This oil container allows for the expansion and contraction of the transformer oil with fluctuating temperatures; it is positioned within the housing and contains the high-voltage subassembly, including the transformer.
For X-ray machines generally, it is important for the tube head assembly to be of compact and lightweight design. It is also desirable that the tube head assembly be constructed within a reliable oil-tight enclosure which will remain oil-tight over the entire life of the tube head assembly. It is also important that the dielectric strength between the various high-voltage components and between the components and ground potential be effective to avoid arcing.
Relatively recent developments in regulated X-ray tube power supplies which operate at high frequency have been advantageously adapted to dental X-ray machines and similar machines within the same class (e.g., portable X-ray units). Modern power supplies which are capable of producing low ripple high voltage power provide a relatively high X-ray energy output per unit of time, thereby reducing the exposure time required to produce diagnostically significant radiographs. U.S. Pat. No. 4,350,891 discloses an exemplary such power supply and associated logic and control circuitry. The disclosure of U.S. Pat. No. 4,350,891 is incorporated herein by reference for its disclosure of a lightweight high frequency power supply for dental systems capable of producing both voltage and current controlled power at low ripple for efficient X-ray tube operation. The power supply illustrated by FIGS. 1, 2 and 4 of that patent is illustrative of the kind of power supply useful for the class of X-ray tube head assemblies with which this invention is concerned. The block diagram of FIG. 3 of U.S. Pat. No. 4,350,891 is also incorporated by reference for its illustration of an X-ray system embodied by X-ray machines of the class to which this invention belongs.
Housings for dental X-ray tube head assemblies have typically been fabricated from either aluminum castings or sheet metal. The requirement for highly effective seals to prevent oil leaks has been recognized. The effective sealing of cooling oil within the tube head of necessity must allow for the expansion and contraction of the contained oil as the oil is heated or cools as a consequence of operation of the X-ray machine. Accordingly, it is essential in X-rays machines of this type that the tube head incorporate means for preventing excess pressure from building up within the interior of the tube head assembly. Several expedients are in use currently, but none is entirely satisfactory.
According to some designs, a bellows filled with air is mounted inside the tube head housing and is vented through the housing to the atmosphere. As the oil contained by the housing expands, it displaces air from the bellows. As the oil cools and contracts, the bellows returns to its normal volume, thereby drawing in ambient air. Certain other tube heads are constructed in a fashion which permits a resilient oil container to expand under the influence of expanding oil and to contract to a characteristic volume when the oil cools. The resilient portion of the container consists of a rubber sleeve mounted between metal ends, being secured to each end by a hose clamp. Other tube head assemblies provide a resilient diaphragm sealing a hole in the housing, thereby permitting an increase in internal volume within the housing as the diaphragm is urged into the hole.
All tube head assemblies require X-ray shielding. Lead is typically positioned either inside or outside the housing, but is preferably placed outside to avoid contact with oil. In the event that flakes of lead find their way into the oil, they can migrate into contact with the internal components of the high-voltage subassembly, creating a high-voltage discharge inside the tube head.
There remains a need for a tube head assembly of improved reliability with respect to the provision of fluctuating volume to accommodate the expansion and contraction of cooling oil. There also remains a need for improved dielectric strength properties between components of the high-voltage subassembly and outside ground potential. It would also be highly desirable to provide a tube head assembly with a decreased volume of cooling oil, thereby reducing the weight of the tube head assembly.